As a matter of definition, an "optical image bar" comprises an array of optical pixel emitters or pixel control elements (collectively referred to hereinafter as "pixel generators") for converting a spatial pattern, which usually is represented by the information content of electrical input signals, into a corresponding optical exposure pattern. Although there are a variety of applications for these devices in several different fields, a significant portion of the effort and expense that have been devoted to their development has been directed toward their application to electrophotographic printing, where they may prove to be a relatively low cost and reliable alternative to the flying spot raster scanners which have dominated that field since its inception. Optical displays and optical memories may also benefit from the use of these image bars, but those applications are of secondary interest at this time.
Optical image bars have spatially distinct pixel generators, so they belong to a broader class of devices, which will be referred to herein as "discrete image bars" to provide a term of sufficient breadth to cover electomechanical devices having similar characteristics. For example, there are multi-stylii impact and electrostatographic printheads, as well as multinozzle ink jet arrays, which are embraced by the term "discrete image bar" as used herein.
Image bars embodying EO TIR (electrooptic total internal reflection) spatial light modulators are particularly interesting, so the more detailed aspects of this disclosure are directed toward them. In keeping with the teachings of a commonly assigned U.S. Pat. No. 4,396,252 of W. D. Turner, which issued Aug. 2, 1983 on "Proximity Coupled Electro-Optic Devices," such a modulator characteristically has a plurality of parallel, individually addressable electrodes which are supported on or closely adjacent a reflective surface of an optically transparent electrooptic (EO) element, such as a lithium niobate (LiNbO.sub.3) crystal. Typically, the electrodes are longitudinally aligned with the optical axis of the modulator and are laterally distributed widthwise of the EO element on generally equidistant centers.
To operate such a modulator, substantially the full width of its EO element is illuminated by a tranversely collimated light beam. This sheet-like light beam is incident on the EO element at a near grazing angle of incidence with respect to its aforementioned reflective surface, so the light beam totally internally reflects from that surface while propagating through the EO element. For modulating the light beam, voltage values representing input data samples (e.g., the pixel values for a given line of an image) are applied to the invidually addressable electrodes of the modulator, thereby producing corresponding fringe electric fields. These localized fringe fields penetrate into the EO element, so they locally vary its refractive index. As a result, the phase front and, in some embodiments, the polarization of the light beam are spatially modulated in accordance with the input data sample values as the light beam propagates through the EO element. Typically, the light beam is brought to a wedge shaped focus on the reflective surface of the EO element to increase the efficiency of the electrooptic interaction. Moreover, the voltage values of the input data samples generally vary as a function of time, such as in accordance with the pixel patterns for successive lines of a two dimensional image, so the modulation imposed on the light beam usually is a time dependent function.
For image bar applications of EO TIR spatial light modulators, prior proposals generally have prescribed Schlieren imaging optics for imaging or "reading out" the modulator onto its output image plane. The frequency plane filtering of a Schlieren imaging system effectively transforms the spatially modulated output radiation of the modulator into a corresponding intensity distribution, but there are embodiments in which a polarization analyzer may be used alone or in combination with a Schlieren stop to perform that function. Thus, as a matter of definition, the term "electrooptic image bar" refers to all image bars which embody electrooptic spatial light modulators, regardless of whether the modulators are read out by spatial filtering and/or by polarization filtering. Indeed, the broader aspects of this invention may be applied to other types of "optical image bars," such as to light emitting diode (LED) arrays, and even to non-optical "discrete image bars," such as the aforementioned electro-mechanical printheads and ink jet arrays. For that reason, a hierarchy of descriptors (i.e., in increasing order of specificity, "discrete," "optical," "electrooptic" and "EO TIR") are used herein to define the term "image bar" with varying degrees of breadth.
There have been several significant developments which have reduced the cost and increased the reliability of EO TIR spatial light modulators. Among the more noteworthy improvements for image bar applications of there devices are a "differential encoding" technique that is described in a commonly assigned U.S. Pat. No. 4,450,459 of W. D. Turner et al., which issued May 22, 1984 on "Differential Encoding for Fringe Field Response Electro-Optic Line Printers," and an electrical interconnect strategy that is described in a commonly assigned U.S. Pat. No. 4,367,925 of R. A. Sprague et al., which issued Jan. 11, 1983 on "Integrated Electronics for Proximity Coupled Electro-Optic Devices." Briefly, it has been shown that the number of electrodes which such a modulator requires can be reduced by a factor of almost two, without sacrificing imaging resolution, if the input data samples are differentially encoded on a line-by-line basis prior to being applied to the modulator. Furthermore, it has been demonstrated that more or less conventional VLSI circuit technology can be employed to integrate the modulator electrodes with their addressing and drive electronics, thereby facilitating the orderly and reliable distribution of data samples to the relatively large number of electrodes which ordinarily are required for reasonably high resolution imaging.
Experience suggests that the manufacturing yield of high resolution EO TIR spatial light modulators could be increased if provision were made to compensate for the localized defects which these devices occasionally exhibit. Open circuited electrodes and inter-electrode short circuits are two of the more common defects that affect such modulators. Unfortunately, these electrical defects seldom can be ignored in practice because they caused a localized loss of modulation control and, therefore, tend to produce readily observable image defects.
Of course, other discrete image bars may suffer from similar imperfections. For example, LED image bars may include faulty emitters, ink jet arrays may contain unreliable nozzles, and matrix configured stylus arrays may have defective stylii. Therefore, even though the primary emphasis of this invention is on relieving the defect tolerance specification for EO TIR image bars, it will be evident that the broader aspects of the invention may be employed to increase the yield and/or improve the performance of other types of discrete image bars. The principal requirement for application of this invention is that the defective pixel generators of the image bar are capable of being disabled, so that the imaging errors they produce can be corrected by overwriting them utilizing other, non-defective pixel generators. Pixel generators which fail in an enabled or "on state" usually are intolerable because they tend to produce "hard" or uncorrectable imaging errors. For that reason, a "defective pixel generator" is hereby defined as being a permanently disabled pixel generator.
As will be understood, a pixel generator may be "permanently disabled" electrically or mechanically, including by means of an opaque mask in the case of an optical pixel generator. Moreover, the defective pixel generators of an image bar may be identified through the use of a pre-test procedure performed, for example, while the image bar is going through final inspection, or they may detected dynamically by more or less routinely examining the image output of the image bar to identify any pixel generators which cease to function properly during operation.
A favored implementation of this invention involves a two step imaging process which may be employed to compensate for defective pixel generators located anywhere on a discrete image bar, provided that the image bar is designed to have a sufficient number of extra pixel generators (as more fully described hereinbelow) to achieve redundant addressing of all pixel positions across the full width of an imaging field of the image bar as a result of a single step shift in the lateral positioning of its pixel generators relative to its imaging field. Excessively large numbers of neighboring or near neighboring defective pixel generators are incompatible with this two step process because of the limited number of extra pixel generators that are available, but the number of extra pixel generators may be increased or decreased more or less at will while the image bar is being designed.